The advantages of screens that are permeable to sound waves are known, in that they enable sound sources to be placed behind the screen, thereby achieving a perceptible cohesion between the projected image and the sound (whether the image is front-projected or back-projected).
Those advantages are described in particular in the document FR 2810122 (CONGARD) and its counterparts EP-1162499 and U.S. Pat. No. 6,552,847.
Disposing the sound sources, in practice loudspeakers, behind a screen in this way is routine in movie theaters, where the screen is conventionally perforated with small holes intended to enable sound to pass through.
In video applications, and in domestic applications in particular, the visibility of the holes is often increased by the proximity of the spectators to the screen. The size and spacing of the holes are conventionally determined by the tools used to produce them, independently of the size of the screen.
Certain manufacturers have recently developed tools for reducing the size and spacing of the perforations whilst preserving a substantially constant ratio of the perforated area to the total area of the screen.
Screens using “woven” technology are also used for these applications.
Most recent video projectors are of the fixed pixel matrix type, using the digital light processor (DLP) or liquid crystal display (LCD) technique, for example. These fixed pixel projectors divide the video image into individual elements each of which is assigned a color and a value.
Projectors of the above kind project an image in which the rows of pixels are visible under certain conditions, especially if the image is viewed from a distance less than the recommended distance. The rows of pixels are in particular apparent as orthogonal alignments of the spaces between pixels parallel to the edges of the image.
In practice, it is found that the order of magnitude of the spacing between the holes in perforated video screens and the dimensions of a projected pixel are often similar. This results in visible interference between two periodic alignments, producing unwanted Moiré effects. Disposing the holes in a quincunx arrangement to reduce their visibility has already been proposed, but has achieved mixed results.
One makeshift solution is to adjust the distance between the projector and the projection screen, which varies the size of the projected image and therefore the size of the pixels, to find positions that generate the least Moiré effect. The results are mixed and do not guarantee that changing projector, for example for one with a higher definition and therefore a different pixel size, will not generate Moiré phenomena again.
These unwanted effects are also encountered with woven screens. When the structure of the canvas causes spaces or projected shadow effects to appear, the periodic alignment thereof generates the same unwanted Moiré effects as in perforated video screens.
Technologies developed with the object of obtaining a good compromise between image quality and permeability to sound waves that do not take account of the risk of the effects of resonance with the fixed matrix type technology (Moiré effects), have therefore proven to be incapable of achieving really good results.
In the situation where the flatness of the screen and/or its orthogonality to the projection axis are not perfect, which is the case in many installations in practice, the Moiré effects are much worse, whence even worse results.
For these reasons, screens that are permeable to sound are at present considered to be problematical in terms of compatibility with fixed pixel projectors, i.e. in practice with the great majority of projectors available, in particular for video projection.